The invention relates generally to circuits and devices that produce a precise and stable DC signal, and more specifically, to temperature compensated bandgap reference circuits.
Virtually all systems that manipulate analog, digital or mixed signals, such as analog-to-digital and digital-to-analog converters, rely on at least one reference voltage as a starting point for all other operations in the system. Not only must a reference voltage be reproducible every time the circuit is powered up, the reference voltage must remain relatively unchanged with variations in fabrication process, operating temperature, and supply voltage.
A conventional technique for realizing a reference voltage uses the semiconductor bandgap reference circuit (also known as a bandgap reference). As explained in detail below, a bandgap reference relies on the predictable variation with temperature of the bandgap energy of the underlying semiconductor material. A practical way to obtain the behavior of the bandgap energy of a semiconductor material is to measure the "bandgap voltage" across a forward biased semiconductor P-N junction (diode) device. Although loosely referred to here as a diode, other devices such as transistors are also typically used to obtain the necessary P-N junction. For example, a conventional way to obtain a bandgap voltage is to diode-connect a bipolar junction transistor (BJT) such that the base to emitter voltage drop V.sub.be is the voltage that exhibits bandgap behavior. The term V.sub.be historically originated with BJT-based bandgap reference circuits. In the remaining discussion, however, V.sub.be is used to refer to any suitable diode-like element that exhibits a diode voltage drop.
FIG. 1 illustrates the extrapolated variation of V.sub.be with temperature for two devices having the same emitter current but different current areas (and hence different current densities). If it were possible to generate a voltage that increased proportionally with temperature at the same rate at which V.sub.be of a given transistor decreased, then the sum of the two voltages will be constant and equal to the bandgap voltage of approximately 1.205 volts, a physical constant. Therein lies the principle behind a bandgap reference.
A conventional bandgap reference 200 that attempts to implement the above principle is illustrated in FIG. 2. The circuit 200 essentially operates as a feedback control loop to maintain the two input nodes of amplifier 217 at approximately the same potential in the steady state. In so doing, the circuit 200 amplifies the difference .DELTA.V.sub.be between the voltages across diodes D.sub.1 and D.sub.2 which are operating at different values of current density due to their different cross-sectional areas. The difference .DELTA.V.sub.be will have a positive temperature coefficient, i.e., a rising slope as a function of temperature, as shown by the required compensation voltage line in FIG. 1, and will typically be several times smaller than the negative temperature coefficient V.sub.be. If the currents in the two unequal area diodes D.sub.1 and D.sub.2 are assumed to be the same in the steady state, and R.sub.2 is set equal to R.sub.3 for easy manipulation of the numbers, then the following equation may be derived by one skilled in the art: EQU V.sub.out =.DELTA.V.sub.be (R.sub.2 /R.sub.1)+V.sub.be
where .DELTA.V.sub.be =V.sub.D1 -V.sub.D2, V.sub.be =V.sub.D1. The ratio R.sub.1 /R.sub.2 is then selected as a gain factor to give V.sub.out approximately equal to the zero Kelvin bandgap energy in electron volts of silicon, i.e., 1.205 volts. Thus, the bandgap principle introduced above is implemented with V.sub.out being the temperature compensated reference voltage.
The bandgap reference 200 is an effective technique for obtaining a reference voltage of approximately 1.2 volts given a supply voltage of a few volts. The last 20 years, however, has seen a steady reduction in the supply voltage used for commercial electronic systems. Older systems typically operated based on a 5 volt supply, while many modern electronic systems that include very dense integrated circuits (ICs) now operate at approximately 3 volts. Electronic systems of the future will need to operate with even lower supply voltages of 1.5 volts or less. The lower headroom is required to maintain the reliability of future ICs by reducing their power densities. Lower supply voltages also reduce total power requirements thereby permitting extended operation time for portable electronics that use batteries. Furthermore, circuits that can operate with low supply voltages can be made compatible with the lower output of solar cells, thereby contributing to a cleaner environment.
The topology of bandgap reference 200 in FIG. 2, however, may require relatively high headroom in a supply voltage of a few volts or greater with respect to ground. Moreover, the reference output V.sub.out lies typically between 1.2 and 1.3 volts, clearly unsuitable for systems having a 1.5 volts supply. Thus, to meet the challenge of such systems, there is a need to develop a low cost voltage reference circuit that can operate with supply voltages of 1.5 volts or less and that provides a reference output well below 1 volt.